An eccentric oscillating-type gear transmission is disclosed in Japanese Patent Publication No. 2002-106650 (Patent Document 1). This gear transmission comprises an internal gear and an eccentrically-rotating external gear meshing with the internal gear. The external gear is supported on a carrier so that it can eccentrically rotate. The carrier is rotatably supported by the internal gear. The axis of the carrier is coaxial with the axis of the internal gear. A crankshaft is rotatably supported on the carrier so as to be parallel to the axis of the carrier. An eccentric body is formed on the crankshaft and the eccentric body is engaged with the external gear. When a motor rotates the crankshaft, the external gear rotates eccentrically. The number of teeth of the internal gear and the number of teeth of the external gear differ. Consequently, when the external gear rotates eccentrically, the carrier rotates, together with the external gear, relative to the internal gear.
The gear transmission of Patent Document 1 further comprises a gear 21, an input gear 17 and a gear 11b. The gear 21 is attached to one end of the crankshaft. The input gear 17 meshes with the gear 21. The gear 11b meshes with the input gear 17. The gear 11b is attached to an output shaft of a motor. Torque from the motor is transmitted to the crankshaft via the gear 11b, the input gear 17 and the gear 21.